This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-100757, filed on Apr. 3, 2002; the entire contents of which are incorporated herein by reference.
The present invention relates to an X-ray imaging apparatus for collecting a subtraction image by photographing a mask image and a contrast image while an arm for oppositely supporting an X-ray tube and an X-ray detector with a subject between is reciprocated and rotated over the range of a predetermined angle around the subject.
A DSA (Digital Subtraction Angiography) image is collected when a blood vessel system, the blood vessels of internal organs, etc. are diagnosed. In a procedure for collecting this DSA image, the mask image is first photographed and is once stored to a memory before a contrast medium is injected. Next, the contrast medium is injected into the subject, and the contrast image is photographed and is also stored to the memory. Then, the mask image and the contrast image in the same position of the subject are read from the memory and both the images are subtraction-processed. Thus, the background such as bones, etc. is removed so that the DSA image left with respect to only a blood vessel image injecting the contrast medium thereinto is obtained.
The digital subtraction rotational angiography is known as a technique for obtaining a solid DSA image of the region of interest as well as the DSA image of a planar shape. In this technique, for example, the X-ray tube and the X-ray detector are oppositely supported at both ends of the arm formed in an arc shape or a C-shape, and an X-ray image is continuously photographed by rotating the arm around the subject rested between this X-ray tube and the X-ray detector. In such rotation photographing, the mask image before the injection of the contrast medium, and the contrast image after the injection of the contrast medium are photographed at the same angle, and a contrast blood vessel, etc. are displayed by subtraction-processing both the images. Thus, an image suitable for the diagnosis can be provided by easily grasping a solid shape.
The schematic construction of a conventional X-ray imaging apparatus for performing such digital subtraction rotational angiography is shown by a systematic view in FIG. 1.
As shown in FIG. 1, the conventional X-ray imaging apparatus has an X-ray tube 1 as an X-ray generator for irradiating an X-ray, an X-ray detector 2 for detecting this X-ray, an arm 3 for oppositely supporting the X-ray tube 1 and the X-ray detector 2 and formed in e.g., a C-shape, an arm driving unit 4 for holding the arm 3 and rotating this arm 3 with its holding axis as a center, and an angle detector 5 for detecting the rotation angle of the arm 3. Further, the conventional X-ray imaging apparatus has an A/D converter 6 for converting an image signal output from the X-ray detector 2 to a digital signal. The apparatus further includes an image memory 7 for recording the image signal converted to the digital signal through the A/D converter 6 and a subtraction module 8 for subtraction-processing plural image signals read from the image memory 7. The apparatus also has a D/A converter 9 for converting the image signal obtained by the subtraction processing in the subtraction module 8 to an analog signal, a display unit 10 for displaying an output of the D/A converter 9, a main controller 11, and an input device 12. The main controller 11 has an central processing unit (CPU) and a memory for controlling the irradiation of the X-ray from the X-ray tube 1, rotation control of the arm 3 using the arm driving unit 4, fetch of the image signal to the image memory 7 and reading of this image signal, subtraction processing of the image signal in the subtraction module 8, etc. The input device 12 has a keyboard, a mouse, a track ball, etc. for suitably inputting a set value, etc. to the main controller 11 by an operator. Further, the conventional X-ray imaging apparatus has a bed 13 for locating the subject placed on a tabletop between the X-ray tube 1 and the X-ray detector 2.
The X-ray detector 2 generally uses a type in which the X-ray image is converted to a visible light image by an image intensifier (hereinafter briefly called I.I.) and this visible light image is photographed by a television camera through an optical system for controlling the transmitting amount of the visible light image formed on the output fluorescent screen of the I.I. However, the X-ray detector 2 may be also constructed by a flat panel detector (hereinafter briefly called FPD) recently practically used and formed by a semiconductor array in which a switching element and a capacitor formed on e.g., a glass substrate are covered with a photoelectrically conductive film, etc. for converting the X-ray to an electric charge, etc. Since the output of this FPD is a digital signal, no A/D converter 6 is required when the FPD is used.
The procedure for executing the digital subtraction rotational angiography by such an X-ray imaging apparatus is as follows.
First, while the arm 3 is rotated over the angle range registered in advance around the subject lying on the bed 13 by giving the instructions of a photographing start from an operator through the input device 12, the X-ray is irradiated from the X-ray tube 1 and an X-ray photograph is taken. Thus, the image detected by the X-ray detector 2 is stored to the image memory 7. At this time, the above image is collected in the same angle range of the arm 3 before and after the contrast medium is injected. As a result, the mask image and the contrast image are obtained every angular position.
Next, in the subtraction module 8, the contrast image and the mask image collected at the same angle as this contrast image are subtraction-processed. This processing is continuously performed at each angle, and its result is displayed in the display unit 10 so that the regeneration of a rotation DSA image is realized.
An image collecting pattern is divided into four kinds of typical patterns as shown in FIG. 2 by the combination of rotating directions of the arm 3 at the collecting time of the mask image and the contrast image.
Namely, FIG. 2(A) shows a pattern in which the mask image is collected by a mask sequence from a rotation start position of the arm 3 to a rotation end position, and the arm 3 is subsequently returned from the rotation end position to the rotation start position and the contrast image is collected by a contrast sequence during this return. This pattern is called an MC mode.
FIG. 2(B) shows a pattern in which the mask image is collected by the mask sequence from the rotation start position of the arm 3 to the rotation end position, and a return operation for returning the arm 3 from the rotation end position to the rotation start position is then performed, and no X-ray is irradiated (no photographing operation is performed) during this return operation, and the arm 3 is subsequently again returned from the rotation start position, and the contrast image is collected by the contrast sequence while the arm 3 reaches the rotation end position from the rotation start position. This pattern is called an MRC mode.
In such MC and MRC modes, the X-ray is irradiated every predetermined angle in the predetermined rotation range of the arm 3, and the mask image and the contrast image are collected. The rotation DSA image is obtained by subtraction-processing both the mask and contrast images respectively photographed in the same angle position.
In contrast to this, similar to the MC mode, FIG. 2(C) shows a pattern in which the mask image is collected by the mask sequence from the rotation start position of the arm 3 to the rotation end position, and the contrast image is subsequently collected by the contrast sequence (a first contrast sequence) during the return of the arm 3 from the rotation end position to the rotation start position, and the contrast image is again collected by again returning the arm 3 from the rotation start position to the rotation end position by the contrast sequence (a second contrast sequence) during this return. This pattern is called an MCC mode.
Further, similar to the MRC mode, FIG. 2(D) shows the following pattern. Namely, in this pattern, the mask image is collected by the mask sequence from the rotation start position of the arm 3 to the rotation end position. Thereafter, a return operation for returning the arm 3 from the rotation end position to the rotation start position is performed. No X-ray is irradiated (no photographing or image acquisition operation is performed) during this return operation, and the arm 3 is subsequently again returned from the rotation start position. While the arm 3 reaches the rotation end position from the rotation start position, the contrast image is collected by the contrast sequence (a first contrast sequence), and the contrast image is again collected by again returning the arm 3 from the rotation end position to the rotation start position by the contrast sequence (a second contrast sequence) during this return. This pattern is called an MRCC mode.
Since the second contrast image is thus collected by the second contrast sequence in the MCC mode and the MRCC mode, the DSA image using the second contrast image can be also obtained. Accordingly, the MCC and MRCC modes have characters particularly effective to observe the degree of a blood flow in static venation of the injected contrast medium.
Next, the relation of a time change in the rotation angle of the arm 3 for collecting the mask image and the contrast image and the subtraction processing will be explained.
FIG. 3 is a typical view shown to explain an image collecting sequence in the MCC mode. Namely, when the arm 3 is set to be reciprocated and rotated between angles A and B, the interval between angles A and Axe2x80x2 is an area for accelerating and decelerating the rotational speed of the arm 3. The interval between angles Bxe2x80x2 and B is also an area for decelerating and accelerating the rotational speed. These areas are called acceleration/deceleration areas. The interval between the angles Axe2x80x2 and Bxe2x80x2 is a constant velocity area in which the rotational speed of the arm 3 is constant. Accordingly, when the rotation of the arm 3 is set to be started from the angle A, the arm 3 is accelerated in an area R1, and is rotated at a constant velocity in an area R2, and is further decelerated in an area R3. When the arm 3 reaches the angle B, the arm 3 is instantaneously stopped and the rotation direction of the arm 3 is then inverted and the arm 3 is accelerated from the angle B in an area R4 . A reciprocating rotation operation is performed by repeating such operations.
When timing of the image collection in such a rotation angle range is seen in time, the mask image is collected in the acceleration/deceleration area R1 (acceleration), the constant velocity area R2 and the acceleration/deceleration area R3 (deceleration). The first contrast image is collected in the acceleration/deceleration area R4 (acceleration), a constant velocity area R5 and an acceleration/deceleration area R6 (deceleration). Further, the second contrast image is collected in an acceleration/deceleration area R7 (acceleration), a constant velocity area R8 and an acceleration/deceleration area R9 (deceleration). The mask image collected in the acceleration/deceleration area R1 (acceleration) is subtraction-processed with respect to each of the first contrast image collected in the acceleration/deceleration area R6 (deceleration) and the second contrast image collected in the acceleration/deceleration area R7 (acceleration). The mask image collected in the constant velocity area R2 is similarly subtraction-processed with respect to each of the first and second contrast images collected in the constant velocity areas R5, R8. Further, the mask image collected in the acceleration/deceleration area R3 (deceleration) is subtraction-processed with respect to each of the first contrast image collected in the acceleration/deceleration area R4 (acceleration) and the second contrast image collected in the acceleration/deceleration area R9 (deceleration). Thus, the rotation DSA image is obtained by these subtraction processings.
FIG. 4 is similarly a typical view shown to explain the image collecting sequence in the MRCC mode. The arm 3 is reciprocated and rotated between angles A and B. The interval between angles A and Axe2x80x2 and the interval between angles Bxe2x80x2 and B are acceleration/deceleration areas. The interval between the angles Axe2x80x2 and Bxe2x80x2 is a constant velocity area. In this case, similar to the MCC mode, the mask image is collected in the acceleration/deceleration area R1 (acceleration), the constant velocity area R2 and the acceleration/deceleration area R3 (deceleration). However, only a return operation of the arm 3 is performed and no photographing operation is performed in the acceleration/deceleration area R4, the constant velocity area R5 and the acceleration/deceleration area R6. The first contrast image is collected in the acceleration/deceleration area R7 (acceleration), the constant velocity area R8 and the acceleration/deceleration area R9 (deceleration). The second contrast image is subsequently collected in an acceleration/deceleration area R10 (acceleration), a constant velocity area R11 and an acceleration/deceleration area R12 (deceleration).
Further, the mask image collected in the acceleration/deceleration area R1 (acceleration) is subtraction-processed with respect to each of the first contrast image collected in the acceleration/deceleration area R7 (acceleration) and the second contrast image collected in the acceleration/deceleration area R12 (deceleration) to obtain the rotation DSA image later. The mask image collected in the constant velocity area R2 is similarly subtraction-processed with respect to each of the first and second contrast images collected in the constant velocity areas R8, R11. Further, the mask image collected in the acceleration/deceleration area R3 (deceleration) is subtraction-processed with respect to each of the first contrast image collected in the acceleration/deceleration area R9 (deceleration) and the second contrast image collected in the acceleration/deceleration area R10 (acceleration).
The MC mode is a case in which no second contrast image is photographed (there is no photograph from R7 and R9) in the MCC mode. The MRC mode is a case in which no second contrast image is photographed (there is no photograph from R10 to R12) in the MRCC mode. Accordingly, since the subtraction processing relating to these patterns can be easily guessed from the above explanation, its explanation is omitted.
In the digital subtraction rotational angiography, an acceleration/deceleration area having no constant rotational speed of the arm 3 always exists just after the rotation start of the arm 3 and just before the rotation stoppage.
Further, when the mask image and the contrast image are subtraction-processed in the above respective patterns except for the MRC mode, there is a case in which the data of in conformity of the rotating direction of the arm 3 are subtraction-processed particularly in the acceleration/deceleration area. Namely, this case corresponds to areas R1 and R6, areas R3 and R4 in the MCC mode and the MC mode, and areas R1 and R12, areas R3 and R10, etc. in the MRCC mode. In such an acceleration/deceleration area, no vibrating states of the arm 3 at the accelerating and decelerating times are conformed to each other in accordance with conditions of the state of the arm 3 and a spatial position, etc. at a returning point of the arm 3 at the reciprocating operation time. Therefore, even when the mask image and the contrast image collected at the same angle are subtraction-processed, an image shift, i.e., misregistration is generated. Therefore, there was a case in which an artifact was caused in the DSA image.
Further, in a system in which the X-ray is irradiated by generating a trigger every time the arm position during the rotation reaches a determined angle, the collecting rate of image data in the acceleration/deceleration area is reduced in comparison with the collecting rate in the constant velocity area in which the rotational speed is constant. Therefore, when the photographed image is regenerated, there is a disadvantage in that an observation is made such that an image regenerating speed in the acceleration/deceleration area is relatively increased. In contrast to this, when the photographed image is regenerated in conformity with the real time, there is a disadvantage in that the image is regenerated as an unnatural image having no frame since there are no image data. Thus, in the acceleration/deceleration area, no situation of the actual blood vessel contrast could be correctly represented in the direction of a time axis.
With respect to such disadvantages, it is also considered that the image is collected only in the constant velocity area without setting the acceleration/deceleration area to an object of the image collection. However, when this method is applied to an image collecting sequence such as the MCC mode and the MRCC mode in which the contrast sequence is returned, the photographing is temporarily interrupted in the returning area. Therefore, discontinuity in time is caused so that this method is not suitable for the observation of a flow degree of the contrast medium.
The X-ray imaging apparatus of the present invention is made in consideration of the above situation, and can reduce misregistration and help to prevent the generation of the artifact. Further, when the DSA image is regenerated in real time, this X-ray imaging apparatus can regenerate the DSA image as a more natural image having no feeling of physical disorder.
According to one aspect of the present invention, the present invention resides in an X-ray imaging apparatus comprising an X-ray generator for irradiating an X-ray to a subject; an X-ray detector for detecting the X-ray irradiated from the X-ray generator and transmitted through the subject; an arc arm for oppositely supporting the X-ray generator and the X-ray detector through the subject; an arm driving unit for rotating the arm around the subject; image acquiring components for collecting a mask image before the injection of a contrast medium into the subject and a contrast image after the injection of the contrast medium by the X-ray irradiated from the X-ray generator during the rotating operation of the arm, and obtaining a subtraction image from the mask image and the contrast image in the same position; and timing control components for controlling irradiation timings of the X-ray with respect to the rotation angle of the arm in a constant velocity area for approximately rotating the arm at a constant velocity, and an acceleration/deceleration area near a start position or a stopping position of the arm such that these irradiation timings in the constant velocity are and the acceleration/deceleration area are different from each other.
Additional aspects of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The aspects of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out herein after.